LCD devices are commonly used as displays for compact electronic apparatuses. This is not only because they provide good quality images with little power consumption, but also because they are very thin. The liquid crystal layer in an LCD device does not emit any light beams itself. The liquid crystal layer has to be lit by a light source so as to clearly and sharply display text and images. Thus, a backlight module is generally needed for an LCD device.
A standard backlight module has no means for controlling its brightness. Therefore a viewer may see displayed images of the LCD device either easily or with difficulty, depending on the intensity of ambient light beams. Thus a better type of backlight module has been developed, with the backlight module being able to adjust its brightness according to the intensity of ambient light beams.
Referring to FIG. 2, this is a schematic, block diagram of a backlight module being capable of adjusting brightness. The backlight module 10 includes a power source 11, an ambient light detector 13, a control circuit 15, and a light source 17.
The power source 11 may be a battery, and provides a power to the control circuit 15 to drive the light source 17. The ambient light detector 13 is used to detect an intensity of ambient light beams, and accordingly generate a signal. The signal is supplied to the control circuit 15. The control circuit 15 generates a control signal according to the signal transmitted from the ambient light detector 13, and automatically modulates the light source 17 to emit light beams with desired brightness.
An operation process of the backlight module 10 includes the following steps. Firstly, the backlight module 10 is initialized after being supplied power, and the control circuit 15 generates an initial driving voltage/current to drive the light source 17. Secondly, the ambient light detector 13 detects variations of the intensity of the ambient light beams, generates a signal corresponding to the variations, and provides the signal to the control circuit 15. Thirdly, the control circuit 15 modulates the driving voltage/current according to the signal, generates a modulated driving voltage/current, and then outputs the modulated driving voltage/current to the light source 17. That is, when the intensity of the ambient light beams is great, the control circuit 15 generates a great driving voltage/current to drive the light source 17; when the intensity of the ambient light beams is low, the control circuit 15 generates a low driving voltage/current to drive the light source 17. Then, the light source 17 receives the modulated driving voltage/current and emits light beams accordingly.
However, when the intensity of the ambient light beams increases greatly, the brightness of the backlight module 10 increases greatly according to the increase of the intensity of the ambient light beams. Then, power consumption of the light source 17 increases, which shortens a working lifetime of the power source 11. Therefore, working lifetime of electronic apparatuses such as mobile phone, portable computer installed the backlight module 10, will be shortened, which causes inconvenience for users.
What is needed, therefore, is a driving circuit for a backlight module that can overcome the above-described deficiencies.